This invention relates to array antennas and, more particularly, to such antennas usable to provide communication with a moving vehicle via satellite.
A variety of forms of antennas have been proposed for point-to-point communication via satellite. In such applications, a radio frequency signal is transmitted from a first antenna providing a beam directed at a satellite, the satellite acts as a repeater re-transmitting received signals, and a second antenna directed at the satellite receives a signal replicating the signal as transmitted from the first antenna. The sequence may be reversed to enable reception at the first antenna of a signal representative of a signal transmitted from the second antenna, to provide two-way communication.
In a form of satellite communication system (referred to generally as a SATCOM system), a series of satellites may be maintained in fixed (GEO) synchronous orbit above the equator, with the satellites in spaced positions along an arc within an equatorial plane. The MILSTAR system is an example of such a system. MILSTAR is a military satellite communication system. Its GEO synchronous satellites transmit at 20 GHz and receive at 45 GHz.
Provision of vehicle-mounted antenna systems suitable for communication via such satellites, while the vehicle is in motion, is subject to a number of constraints. The antenna is desirably of relatively small size and reasonable cost. Thus, while a two-dimensional fully electronically scannable phased-array type antenna might be considered, cost would generally be prohibitive and low angle (low elevation) scanning would typically be limited. Additional constraints are requirements for adequate antenna gain, with the largest possible beamwidth to enhance signal capture, but with low sidelobe performance. Low sidelobes are particularly important in order to enable discrimination between signal transmission/reception characteristics (i.e., antenna patterns) of adjacent satellites to avoid interference during signal reception and transmission from a vehicle. Known forms of prior antennas have generally not been capable of meeting all constraints relevant to such applications.
Objects of the present invention are, therefore, to provide new or improved antenna systems suitable for communication via satellite and antenna systems providing one or more of the following capabilities or characteristics:                diamond-type array configuration with reduced diffraction effects;        low sidelobes in principal beam planes;        reduced sidelobe levels relative to a rectangular-type array;        satellite tracking capability from a vehicle moving over terrain;        thin construction with flat-plate subarrays;        ultra-thin flat-plate subarray design;        cost effective design; and        compact size.        